Additional oil recovery by improved miscible displacement



ADDITIONAL 01L RECOVERY BY IMPROVED MISCIBLE DISPLACEMENT Filed Feb. 27.1963 4 Sheets-Sheet 1 VAPOR m on. our

; RESIDUAL couosmszo 2 SOLVENT SOLVENT Q 8 a 0 l a i G, a 35.121222. 1:5 11:11:: H c, VAPOR 1: 1 I u #1 U *iz: a [T a :7 I6 r 1 i I i I IVAPOR passsunz 4 or SOLVENT v n TEMPERATURE or nzsanvom 1 PRESSUREpnessune TEMPERATURE -o ORIGINAL nescnvom TEMPERATURE John R. KyteINVENTOR ATTORNEY Aug. 1, 1967 J. R. KYTE 3,333,632

I ADDITIONAL 0 1L RECOVERY BY IMPROVED MISCIBLE DISPLACEMENT Filsd Feb.27, 1963 r 4 Sheets-Sheet a (LOMPARSON OF CONDENSiNG FREON DRIVE I WITHOTHER RECOVERY METHODS .1. E 70 C E i- 60 2 u u (I Lu 0.

E I E zcs uo w L g 0.5 PV Condensing Solver" 015V. 0 Q 0.07 PVCundansinq 50mm Drivl m 40 n n: 6 Nilmqan-Orlvnn, 0.07 PV CondcnflnqSolvlnl Drivu 9 Liquid solnm on" 6 60- Driven, 0.15 PV Liquid SolvonlDrive wuw'flood V 1 3o Nmugcn DIWI 2o 7 l I I l I O 0.05 030 0.85 0.200.25

DRIVING FLUIDS PRODUCED EXPRESSED AS PV LIQUID FRECN FIG. 3.

John R. Kyte INVENTOR.

ATTORNEY j Aug.*1.- 1967 i J. R. KYTE 3,333,632

ADDITIONAL OIL RECOVERY BY IHPROVED I QISCIBLE DISPLACEMENT Filed Feb'27. 1963 4 Sheets-Sheet 4 WITH OTHER RECOVERY METHODS OIL aecovsnv mPER-CENT or mmm. on. O b o 0 V Niiroqon-Orivon, 0.065 PV CondunsinqSolvnm on" BLOWDOWN LEQEND 0.3 PV Condansing Solvant Drive 0.! PVCcndunsing SoIvnm Dvive Ni'rogm-DflwmQO? RV Liquid Solvent Drlvo 0.2 PvLiquid Solvnni lnllchd Fullomd By Slowdown Liquid Sqlvont Floodwonrfleod Y I Nihoqun Drhn i 3 11 0.05 0J0 I 0J5 H 0.20 0.25 omvmsFLUIDSYPRODUCEDY EXPRESSED AS PV ucum PROPANE John R. Kyte mvmoxi BY wAT ORNEY Patented Aug. 1 1967 The present invention is broadly concernedwith an improved method for increasing the recovery of crude petroleumfrom subsurface reservoirs. More particularly the invention relates tothe recovery of oil by an improved miscible displacement process whichinvolves the injection of a solvent vapor which is readily condensableat reservoir conditions of temperature and pressure. Specifiselect forinjection a vapor which has a critical temperature higher than thereservoir temperature, and which also possesses a vapor pressure atreservoir temperature which is high enough to prevent excessivecondensation. Other essential details of the process are explainedbelow. In the recovery of petroleum from natural reservoirs numerousmethods have been proposed heretofore which involve the injection of abank of miscible fluid which in turn is driven through the reservoir byvarious inert and inexpensive drive fluids, including for example, wateror natural gas. It is well known that for various reasons such priormethods sufier certain disadvantages which prevent any substantialapproach to that degree of success which,

theoretically, can be achieved by miscible displacement. Typically, whena solvent bank is injected it demonstrates a tendency to finger orchannel through the more viscous reservoir crude. Then, when an inertgas such as methane or natural gas is injected after the solvent itreadily fin gers through both the solvent bank and the reservoir oil.As'a result the gas breaks through prematurely at the production wells,causing large volumes of gas and solvent to be produced with relativelyunsatisfactory oil recovery. Because of the adverse viscosity ratio andgravity segregation, such processes frequently recover even lessoil'than conventional waterflooding techniques.

In accordance with the method of this invention a solvent bank isinjected into the reservoir as a vapor. Initially,

the vapor condenses in the vicinity of the injection well- 'bore,thereby forming a bank of liquid, condensed solbank, driving itprogressively farther from the injection well. The reservoir oil is inturn displaced ahead of the condensed vapor bank.

Once the reservoir temperature behind the vapor front is raised a fewdegrees, the injected vapor is permitted to flow a great distancewithout appreciable transfer of heat to the reservoir, until the vaporpasses through the heat front and contacts unheated reservoir.Essentially, the entire available heat content of the vapor is therebypreserved for delayed release at a time and place which most efficientlyadvances the heat front and replenishes the condensate bank, withoutprogressively raising the reservoir temperature to higher and higherlevels near the injection wells.

Preferred solvent-vapors, suitable for injection in accordance with themethod of this invention, include hydro- -cally, it is-essential inaccordance with the invention to gen sulfide, ammonia, propane, sulfurdioxide and sulfur trioxide. A particular solvent for usein flooding agiven reservoir is selected to provide a vapor pressure at reservoirtemperature of at least 50 p.s.i.a., and preferably at least p.s.i.a.The critical temperature of the selected solvent must be higher thanreservoir temperature in order to permit condensation and the release oflatent heat.

As a practical limit, the critical temperature should be at lemt 5 F.above reservoir temperature.

Particularly preferred solvent vapors are hydrogen sultide and sulfurdioxide, because of the increased density imparted by these solvents tothe reservoir crude at the transition zone between the leading bank ofcondensed vapor and the reservoir crude. in accordance with this em- 1bodiment, any vapor which may tend to override the reservoir crude willsaturate the oil at the top of theformation. The solvent-containingreservoir crude is rendered more dense than the remaining reservoir oil.The dense mixture then flows downward from the force of gravity and willbe replaced by undersaturated oil. Thus, any overriding tendency iscorrected because the fresh undersaturated oil will converge upon anyregion of high vapor saturation near the top of the reservoir. Thisvertical mixing of solvent with oil greatly improves the displacementefiiciency of the drive.

a condensed solvent bank. However, a critical distinction exists betweenthe mixed gas-vapor system and the systern of the present invention. Inthe mixed system the condensable portion of the gaseous phase isselectively condensed upon contacting unheated reservoir just ahead ofthe heat front. Thc.nou-condensable gas is then free to finger ahead ofthe condensate bank and create channels through which a portion of thegas-vapor mixture will flow. This results in a deterioration of the mainsolvent bank and ineliicient displacement of reservoir oil.

' A mixture of condensable vapors is suitable only when the condensingproperties of the mixture are substantially the same as a single vapor;i.e., the separate vapors condense in the same proportion as theirconcentrations inthe vapor. A mixture comprising substantial volumesofboth propane and butane vapors is unsuitable, since butane would beselectively condensed, allowing the propane vapor to linger ahead in thesame manner as a non-condensable gas. Propane and ammonia have nearlyidentical vapor pressures at temperatures between 20 and 40 C., andtherefore form a suitable mixture. See the Chemical Engineers Handbook,2nd edition, pp. 2548 and 2549, John H. Perry, editor, McGraw-Hill BookCo., Inc., for a further disclosure of such mixtures.

' It will be apparent to those skilled in the art that the fraction ofinjected vapor which condenses is an important variable in the process.This fraction will depend upon the physical properties of the solvent,upon the heat losses which occur in the reservoir, upon the temperatureof vapor injection, and upon the pressure distribution in the reservoirbetween the input and production wells.

In the operation of a flood in accordance with the invention, it isdesirable-that the pore volume occupied by uncondensed vapor be at leastas great as the volume occupied by the condensed, liquid solvent bank.Preferssatasz 3 ably, the ratio of vapor volume to condensed solventvolume should be at least 4 to l.

Upon condensation at typical reservoir pressures, the injected vaporshrinks to a liquid volume only onetenth to one-thirtieth of its vaporvolume. Accordingly, if 90% of an injected vapor condenses, over theentire history of a hood, the remaining of the vapor occupies a volumeat least as great as the condensed portion. if only 50% of the vaporcondenses, then the remaining vapor occupies a volume 10 to 30 times asgreat as the liquid bank. Therefore, in accordance with this invcntion,a hood is readily completed with a volume of solvent vapor equivalent toonly one-tenth pore volume of liquid solvent, as demonstrated by thelaboratory-scale examples which follow.

FIGURE 1 shows a vertical cross section of a reservoir subjected to thecondensing vapor drive of the invention. Also, the pressure andtemperature gradients established in the reservoir are illustratedgraphically.

FIGURE 2 illustrates the pressure distributionsestablished within areservoir during an early flooding stage and a late flooding stage, whendisplacing reservoir oil with a condensing propane drive.

FIGURE 3 is a comparison of oil recoveries obtained with a condensingFreon drive versus. other recovery methods.

FIGUREA is a similar comparison of a condensing propane drive with otherrecovery methods.

Referring again to FIGURE 1, reservoir 11 is penetratedby input wellbore12 and production wellbore 13. An advanced flooding stage isillustrated, wherein reservoir oil 14 is being displaced by a bank ofliquid solvent 15, which in turn is being driven by solvent vapor 16.Small volumes of condensed solvent 17 are formed behind bank 15, butonly that amount necessary to balance heat losses from the formation.

Curve 18 indicates the pressure distribution between wells whichcorresponds to the flooding stage illustrated in reservoir 11. Note therelatively small pressure drop between the input well and the vaporfront, compared with the sharp drop between the vapor from and theproduction well. A uniform reservoir permeability between wells isassumed; The break in curve 18 is due to the difference between theviscosities of the solvent vapor and the reservoir oil.

Curve 19 illustrates the temperature'distribution es tablished by vaporcondensation between the input well and the leading portion of bank 15.The input vapor temperature is maintained at least 5 F., and preferablyat least 5, above the initial reservoir temperature. Considerably higherinput temperatures may be employed, but the excess sensible heat wouldbe rapidly dissipated near the injection wells, without significantbenefit to the process. Line 20 represents the original reservoirtemperature. Behind the vapor front correspondingpoints on curves 18 and19, such as joined by line 21, represent saturated vapor conditions.That is, the pressure is the vapor pressure characteristic of thesolvent at the given temperature. in the vicinity of an injection well,some departure from this condition is desirable as a means of minimizinginjectivity loss. A superheating of injection vapor will prevent anycondensate from remaining near the injection wellbore to cause a reducedpermeability to vapor.

The process is further illustrated by reference to FIG- URE 2 whichshows calculated pressure distributions within a reservoir for acondensing propane drive with injection and producing wells spaced onethousand feet apart in a four hundred millidarcy sandQ'The logarithmicscale of distances is based on adjacent radial flow systems, each havinga radius of 500 ft. This approximates a convcntional "five-spot pattern"of well spacing. The reservoir is assumed to be at 170 F. initially, anda constant oil production rate of 30 barrels per day per foot of sand ischosen as a convenient example. A higher production rate ass-3, 32.

can be obtained, if desired, by increasing the temperature and pressureof the injected vapor. However, this would reduce the efficiency of theprocess by increasing heat losses.

. below the oil-bearing formation. In this manner a heat bank is movedoutward from the injection well. Most of the injected vapor flowsthrough the heated region without condensing until it contacts coolerregions of the reservoir at the front of the heat bank. Thus acondensing solvent bank is maintained at the heat front, as shown iFIGURE 1.

The temperatures associated with the pressures inthe vapor bank areshown on the pressure distribution curve for an early flooding stage andfor a late flooding stage. That fraction of the reservoir which liesbetween one hundred and nine hundred feet from the injection well mustbe heated only about 1 to 6 F. above the initial reservoir temperature.Since this represents about 96% of the reservoir volume, heat lost fromthe formation will I be very small as compared with conventional heatbank processes in which temperature differentials are sometimes as greatas several hundred degrees.

During the initial stages of a flood, there is of course no benefit tobe obtained from the maintenance of a backpressure at production wells.During the latter stages of a flood, however, back-pressure atproduction wells is usually desirable to maintain the necessary pressurefor condensation at the vapor front. In FIGURE 2 the backpnessuremaintained at the production well is indicated by the point ofintersection of each pressure curve withthe right-hand margin of thechart. As indicated, the back pressure required to provide a constantproduction rate of 30 barrels per day per foot of reservoir sand becomesprogressively greater during the latter stages-of a hood.

The injected solvent is readily recoverable, upon termination of thedrive, by reducing the pressure at the production and/or injectionwells. Preferably, this blowdown" stage of the process is begun when theproduced oil is found to contain substantial volumes of solvent.

A principal advantage of the present method over the conventionalgas'driven solvent bani; is the virtual elimination of the normaltendency for a gas to finger or channel through the reservoir. This isreadily accomplished since vapor fingers will be condensed once theycontact unheated portions of the reservoir ahead of the main vapor bank.Although a finger may eventually form after suiflicient condensation hasoccurred to create a hot spot in the reservoir the lingering tendency isnevertheless opposed by heat transfer considerations. Since heat will betransferred from the hot spot to the surrounding cooler portions of thereservoir, excess condensation will occur where the fingers do formthereby correcting the initial tendency. At the same time the rock inthe vicinity of the fingers will be preheated by this condensation sothat the main vapor bank will continue to travel through these regionsat higher than average velocity and with less than centipoises. Thetests were performed using Freon (dichloroditluoromethane) as thecondensing solvent drive. its

gas and liquid viscosities are 0.012 and 0.26 centipoise, Q

respectively, and it has a density of about 80 pounds per cubic foot.These values are almost identical with the viscosities and densities ofsulfur dioxide.

FIGURE 3 shows a comparison of a condensing Freon drive with otherrecovery methods. Oil recovery in pcrcent of original oil in place isplotted against the quantity of driving fluid produced. The latterparameter is can pressed as pore volume of liquid Freon. For example, inthe nitrogen gas drive the volume of nitrogen produced at atmosphericconditions was assumed to he Freon vapor; then computations were made toconvert this vapor to Freon liquid at room temperature. Admittedly, thisrepresents an arbitrary simplification of the data, but for a comparisonof the various recovery methods this treatment of the data is valid on asemi-quantitative basis.

The uppermost curve in FIGURE 3 shows the recovery for a condensingFreon drive in which the total amount of Freon vapor injected wasequivalent to 0.15 pore volume of liquid. In this experiment the inletend of the metal-mounted core was heated to 100 F. and the outlet endwas held at 72 F. At these temperatures the saturated vapor presure ofFreon is 117 and 73 p.s.i.g., respectively. Freon vapor was introducedinto the core at 100 p.s.i.g. and H30 F. with a back-pressure of 73p.s.i.g. im-

posed at the outlet end. Thus the Freon superheated at the core inlet.

Heating of the core is a necessary control feature in these laboratoryexperiments because of the ditliculty of simulating reservoir conditionswith insulation alone as a means of reducing heat loss. The large heatcapacity of the metal surrounding the core would otherwise cause all theFreon vapor to condense. Heating of the core to estsblish a temperaturegradient along its length does, in efiect,

vapor was slightly "satisfactorily simulate reservoir conditions.

' In the above tests, 87 percent of the original oil in place wasrecevored using a.total amount of Freon equivalent to 0.15 pore volumeof liquid Freon. Of particular interest is the high recovery (80 percentof the original oil in place) obtained with the production of only 0.005pore volume of solvent. When compared with other solvent methods of .oilrecovery for this core, this demonstrates that the condensing solventdrive of the present invention is by far the most eilicient.

Another condensing Freon drive was performed using a volume of solventvapor equivalent to only 0.07 pore volume of liquid. As seen from FIGURE3 the condensing drive using a total of only 0.07 pore volume of liquidFreon is almost as efiicient as the test where 0.15 pore volume. wasinjected. This experiment was performed in the same manner as the firsttest with the exception of injecting less vapor, as indicated, and withthe further modification of maintaining a back-pressure on the core" 4p.s.i. lower than in the first test, to reduce the amount ofcondensation.

A third condensing Freon drive was conducted, involving an interruptionof Freon injection followed by nitrogen injection after about 35 to 40percent of the original oil 'in place had been produced. In this mannerit was sought to. dcterminewhether the condensing solvent system onceestablished, could be driven through the remainder of the rescrvoirwiththe injection of an inert gas. As men from FIGURE 3 this was the leastefiicient oi the three condensing solvent drives tested. Nevertheless,even this modification is an improvement over prior miscibledisplacement processes, and is therefore considered to be an inventiveembodiment.

Similarly, once the condensing vapor drive is established and asubstantial proportion of the original oil in place has been produced,it is possible to follow the solvent vapor with other conventional floodmedia, although not necessarily with equivalent results. For example,the condensing vapor system may be followed by the injection of hotwater, cold water, natural gas, air or steam.

From HGURE 3 it is seen that the recovery characteristic of thecondensing solvent drives is significantly better than either a straightsolvent flood or a gas-driven was recovered by a condensing vapor driveusing 21 volume of propane vapor equivalent to about 0.3 pore volume ofliquid. Eighty-one percent of the original oil in place was recoveredusing a total of 0.1 pore volume propane. The liquid propane drive andthe gas driven propane bank are much less efiicient.

Referring again to FIGURE 3, the condensing Freon drive does show amarked increase in oil recovery over Waterflooding. Moreover, andequally-important, high recovery is obtained with very little productionof the driving fluid. The reason for this increase in recovery or Freoncompared with propane is undoubtedly due to the difference in fluidviscositles of the two materials.

While various embodiments of the invention have been specificallydescribed, other modifications will occur to those skilled in the art,without departing from the spirit of the invention. Accordingly, it iscontemplated that no limitation be placed on the scope of the invention,other than as recited in the appended claims.

What is claimed is:

1. A method for increasing the recovery of oil from a porous sub-surfacereservoir penetrated by an injection well and a production well whichcomprises introducing into said reservoir through said injection well acondensable solvent vapor selected from the group consisting of hydrogensulfide, sulfur dioxide, ammonia, and sulfur' trioxide, under conditionsof temperature and pressure which cause a partial condensation of theinjected vapor upon its approachto thermal equilibrium with thereservoir, thereby forming an oil miscible condensate bank; driving theresulting condensate bank through the reservoir by continuing to injectsaid condensable vapor stream under conditions of temperature andpressure which establish and maintain a vapor bank having a volume atleast as great as the volume of the condensate bank; and producingreservoir oil from said production well.

2. A method as defined by claim 1 wherein said condensable vaporcomprises hydrogen sulfide.

3. A method as defined by claim 1 wherein said c0n W densable vaporcomprises sulfur dioxide.

4. A method as defined by claim 1 v'vhereinsaid condensable vaporcomprises ammonia.

S. A method as defined by claim 1 wherein said condensible vaporcomprises sulfur trioxide.

6. A method for increasing the recovery of oil from a porous surfacereservoir penetrated by an injection well and a production well whichcomprises introducing into said reservoir through said injection well adisplacing medium consisting essentially of a single condensable solventvapor, under conditions of temperature and pressure to cause only apartial condensation of the injected medium upon its approach to thermalequilibrium with the reservoir, thereby forming at; oil-misciblecondensate bank, driving the resulting condensate toward said'productionwell by continuing to inject said condensable vapor stream, thereafterinjecting into the reservoir through said injection well a fluidselected from the group consisting of hot water, cold water, naturalgas, air and and producing reservoir oil from said production steam,well.

7. A method as defined by claim 6 wherein said fluid ar c 9. A method asdefined by claim fiwherein s aid fluid 2,968,350 1/1961 Slobodet a1. 1669 comprises natural gas. 3,101,781 8/1963 Connally. 166-9 10. A methodas defined by claim 6 wherein said fluid comprises air. FOREIGN PATENTS11. 'A method as defin d by claun 6 wherem sald fluld 696524 911953Great Britain.

.compnses steam. 726,712 3/1955 .GreatBritain.

References flied Y UNiTED STATES PATENTS CHARLES E. O'CONNELL, PrimaryExaminer. i- 2,412,765 12/1946 Buddrus etai ass-4X w STEPHENLNOVOSAQEMW', 2,859,818 11/1958 Hail et al 166-42.! X T. A. ZALENSKI,Assistant Examiner. Q}

1. A METHOD FOR INCREASING THE RECOVERY OF OIL FROM A POROUS SUB-SURFACERESERVOIR PENETRATED BY AN INJECTION WELL AND A PRODUCTION WELL WHICHCOMPRISES INTRODUCING INTO SAID RESERVOIR THROUGH SAID INJECTION WELL ACONDENSABLE SOLVENT VAPOR SELECTED FROM THE GROUP CONSISTING OF HYDROGENSULFIDE, SULFUR DIOXIDE, AMMONIA, AND SULFUR TRIOXIDE, UNDER CONDITIONSOF TEMPERATURE AND PRESSURE WHICH CAUSE A PARTIAL CONDENSAITON OF THEINJECTED VAPOR UPON ITS APPROACH TO THERMAL EQUILIBRIUM WITH THERESERVOIR, THEREBY FORMING AN OIL MISCIBLE CONDENSATE BANK; DRIVING THERESULTING CONDENSATE BANK THROUGH THE RESERVOIR BY CONTINUING TO INJECTSAID CONDENSABLE VAPOR STREAM UNDER CONDITIONS OF TEMPERATURE ANDPRESSURE WHICH ESTABLISH AND MAINTAIN A VAPOR BANK HAVING A VOLUME ATLEAST AS GREAT AS THE VOLUME OF THE CONDENSATE BANK; AND PRODUCINGRESERVOIR OIL FROM SAID PRODUCTION WELL.